Segmented addressing is the internal mechanism that combines a segment value and an offset value to create an address. The two parts of an address are represented as
segment:offset
The segment portion is always a 16-bit value. The offset portion is a 16-bit value in 16-bit mode or a 32-bit value in 32-bit mode.
In real mode, the segment value is a physical address that has an arithmetic relationship to the offset value. The segment and offset together create a 20-bit physical address (explained in the next section). Although 20-bit addresses can access up to one megabyte of memory, the operating system on IBMÒ PCs and compatibles uses part of this memory, leaving 640K of memory for programs.